To study the mechanism of chest pain in patients with arteriographically normal coronary arteries, 50 patients underwent great cardiac vein flow, lactate and oxygen content determination at rest and during pacing, and left ventricular end-diastolic pressure measurement immediately after pacing. Those patients experiencing their typical chest pain during pacing had less of a rise in flow and fall in coronary resistance, less lactate consumption, and higher post-pacing left ventricular end-diastolic pressure than those patients without pain. After ergonovine, a larger group demostrated hemodynamic abnormalities similar to patients experiencing chest pain in the control pacing study. No epicardial coronary artery spasm was noted after ergonovine. The data suggest that some patients with chest pain and angiographically normal appearing coronary arteries have dynamic abnormalities of the small coronary arteries or coronary microcirculation, resulting in abnormal vasocilator reserve or vasoconstriction.